1. Field of the Invention
The present general inventive concept relates to an image forming apparatus, and more particularly, to an image forming apparatus to save toner and a method to control the same which can increase the clearness of data (for example, images or characters) on its output material while achieving toner saving.
2. Description of the Related Art
An image forming apparatus such as a printer generally uses halftoning to print images. Halftoning is a method to represent shades of gray using halftone cells, each in the form of a lattice including a number of dots. This method represents a shade of gray using a corresponding number of dots displayed in a halftone cell.
A halftone image is not a continuous tone image like that on a full-color printout of a Dye Diffusion Thermal Transfer (DDTT) printer. However, when viewed at a certain distance, a halftone image is viewed as if it is a continuous image rather than a combination of dots.
Image data input to such an image forming apparatus is represented with 256 gray levels. The image forming apparatus uses the halftoning method since it cannot arbitrarily vary the density of toner applied to a sheet of paper, like a light emitter of a computer monitor or the like which can arbitrarily vary its luminance, and can only determine whether or not to apply toner to a sheet of paper.
Reference will now be made to a conventional image output method for an image forming apparatus.
FIG. 1A illustrates example original image data, FIG. 1B illustrates an enlarged digital image of part of the original image data, FIG. 1C illustrates a halftone table corresponding to the digital image illustrated in FIG. 1B, FIG. 1D illustrates halftoning result values obtained by comparing grayscale values of the digital image illustrated in FIG. 1B with the halftone table illustrated in FIG. 1C, and FIG. 1E illustrates dot images corresponding to the halftoning result values illustrated in FIG. 1D.
As illustrated in FIGS. 1A to 1D, the image forming apparatus compares grayscale values of a digital image of an original image with a halftone table and outputs halftoning result values.
The image forming apparatus represents the halftoning result values by corresponding dot images as illustrated in FIG. 1E and discharges toner to the represented dot images to print the image data.
However, printing the image data in this manner increases the amount of toner consumed. An electro-photographic printer, such as a laser printer, having a toner saving function has been developed and used to reduce toner consumption.
FIG. 2A illustrates halftoning result values of a conventional image forming apparatus having one toner saving function, FIG. 2B illustrates dot images corresponding to the halftoning result values illustrated in FIG. 2A, and FIG. 2C illustrates halftoning result values of a conventional image forming apparatus having another toner saving function. When the user 105 has selected a document for printing, a controller in the printer (i.e., the image forming apparatus) determines whether or not the toner saving function has been selected before printing the document. When the toner saving function has been selected, the controller of the printer supplies a lower voltage than a default voltage to a developer roller.
This reduces the amount of toner provided to a photosensitive drum to allow low-density images to be printed on a sheet of paper as illustrated in FIG. 2C.
In another method to reduce toner consumption, the image forming apparatus omits printing of some pixels at regular intervals of those of the original image data, thereby saving toner.
More specifically, as illustrated in FIGS. 2A and 2B, the image forming apparatus prevents toner from being discharged for even toner lines, thereby saving 50% of the toner.
Although the conventional image forming apparatus having such a toner saving function can reduce toner consumption, it cannot accurately reproduce the original image, thus leading to a reduction in the readability of letters and a failure to provide a printout having sharp details (for example, images or characters).